This invention relates to a novel device and method for determining the flow rate of thermoplastic materials. In particular, this invention relates to a device and method for determining flow rate which utilizes electronic circuitry to monitor the extrusion or displacement of a known volume of a thermoplastic sample and conveniently calculate the flow rate of said sample. More particularly, this invention relates to a novel device and method which automatically and instantaneously computes and displays with high precision the flow rate of thermoplastic samples run in an extrusion plastometer in accordance with American Society of Testing and Materials (ASTM) Method D1238. This invention also relates to other types of melt, liquid, or solution viscosity measurements in which a flow or efflux time is measured and for which a simple computational constant can be derived.
The melt flow rate or "melt index" of thermoplastics as determined by ASTM Method D1238 (Current edition approved Jan. 26, 1979; published February, 1979; originally published as D1238-65T; last previous edition D1238-73--hereby expressly incorporated herein by reference), or foreign counterparts, is universally employed as a specification or inspection property. As a result, the extrusion plastometer is widely used in polymer manufacturing plants, polymer fabrication plants, technical service, research and other laboratory installations the world over. More recently, a variety of plastometer configurations have been developed. Examples of such plastometers are Brown et al. (U.S. Pat. No. 3,807,221), Murphy et al. (U.S. Pat. No. 4,062,225) and Fuxa (U.S. Pat. No. 4,109,516).
In Procedure A of ASTM Method D1238, the operator manually cuts off portions of extrudates at specified time intervals and weighs them on a balance accurate to .+-.0.001 g. This procedure is time consuming, requires an expensive balance as an accessory, and is subject to errors in both collecting extrudates at exact time intervals and in the weighing step. The final result is calculated manually in accordance with the test method. Procedure B of ASTM Method D1238 involves the use of an automatically operated timer to determine the time required to extrude a known volume of polymer. The flow rate is then determined from the following relationship EQU Flow rate=F/t
where F is a numerical factor which takes into account the volume extruded, the size of the piston used to extrude or displace the polymer, and the density of the polymer melt; and t is the time taken to extrude the known volume. Values of F are different for each polymer and are well known in the art. The method still involves a manual calculation of the result and is subject to the potential incorrect entry of the numerical factor each time the calculation is repeated. Precision may also be limited by the type of timing device used. Our invention overcomes the limitations of the currently-used procedures mentioned above through the use of a low cost microprocessor and supporting circuitry.
Previous attempts at an automatic flow rate measuring and method, most notably the Monsanto Automatic Capillary Rheometer, remain limited by the fact that the actual computation of flow rate must be performed each time by the user. In addition, such "automatic" devices are basically little more than sophisticated timers--the user is still required to read the timers and perform the subsequent calculations.
In contrast, the device and method of this invention requires only that the user input the numerical factor characteristic of the particular material being measured. All timing and control functions of the measurement, as well as all computational functions, are subsequently directed by a microprocessor and supporting circuitry. The flow rate is then displayed by means of the output device of choice.
The apparatus of this invention utilizes Procedure B of ASTM Method D1238. In accord with this method, the material to be measured is first preheated for a short period to prepare it for extrusion. A weighted piston of known surface area is then used to force material through a barrel and extrusion plastometer orifice of known dimensions. The measurement is based upon determining the time required to displace a known volume of material.